Well, it's more than that sometimes....
Sometimes the Wah just works in a fequency range that doesn't jibe with the amp. I had a CryBaby that was barely discernable with a certain amp I owned. I had a Morley that with a different amp did nothing but generate white noise.
A really good sounding Wah is one half the sound your going for and one half just sheer functionality with the amp and guitar that you're using.